S&P CNX Nifty is a well diversified 50 stock index accounting for 21 sectors of the economy. It is used for a variety of purposes such as benchmarking fund portfolios, index based derivatives and index funds.

S&P CNX Nifty is owned and managed by India Index Services and Products Ltd. (IISL), which is a joint venture between NSE and CRISIL. IISL is India's first specialised company focused upon the index as a core product. IISL has a Marketing and licensing agreement with Standard & Poor's (S&P), who are world leaders in index services.

S&P CNX Nifty is computed using market capitalization weighted method, wherein the level of the index reflects the total market value of all the stocks in the index relative to a particular base period. The method also takes into account constituent changes in the index and importantly corporate actions such as stock splits, rights, etc without affecting the index value.

Source: NSE

The constituents and the criteria for the selection judge the effectiveness of the index. Selection of the index set is based on the following criteria:

Liquidity (Impact Cost)

For inclusion in the index, the security should have traded at an average impact cost of 0.50% or less during the last six months for 90% of the observations for a basket size of Rs. 2 Crores.

Impact cost is cost of executing a transaction in a security in proportion to the weightage of its market capitalisation as against the index market capitalisation at any point of time. This is the percentage mark up suffered while buying / selling the desired quantity of a security compared to its ideal price (best buy + best sell) / 2

Floating Stock

Companies eligible for inclusion in S&P CNX Nifty should have atleast 10% floating stock. For this purpose, floating stock shall mean stocks which are not held by the promoters and associated entities (where identifiable) of such companies.

Others

a) A company which comes out with a IPO will be eligible for inclusion in the index, if it fulfills the normal eligiblity criteria for the index like impact cost, market capitalisation and floating stock, for a 3 month period instead of a 6 month period.

b) Replacement of Stock from the Index:

A stock may be replaced from an index for the following reasons:

i. Compulsory changes like corporate actions, delisting etc. In such a scenario, the stock having largest market capitalization and satisfying other requirements related to liquidity, turnover and free float will be considered for inclusion.

ii. When a better candidate is available in the replacement pool, which can replace the index stock i.e. the stock with the highest market capitalization in the replacement pool has at least twice the market capitalization of the index stock with the lowest market capitalization.

With respect to (2) above, a maximum of 10% of the index size (number of stocks in the index) may be changed in a calendar year. Changes carried out for (2) above are irrespective of changes, if any, carried out for (1) above.

Source: NSE